It is well known that heat in vehicle braking systems is generally deleterious to the system, both structurally and functionally. It is further well known that aircraft often employ braking systems having brake disc stacks comprised of interleaved discs. The discs are alternately splined to the wheel and torque tube such that alternating rotor and stator discs are present within the stack. A stationary end plate is provided at one end of the stack, while an axially movable pressure plate is provided at the other. The application of force to the pressure plate urges the stator and rotor discs into frictional engagement with each other, providing the braking activity from the aircraft.
Presently known aircraft brake discs are made of steel, carbon, or appropriate composites. Irrespective of the type of material employed, tremendous heat is generated in the brake disc stack during a landing operation. While the brake discs typically begin to cool immediately following landing, the braking operations incident to taxiing generate heat in the stack, retarding the effective cooling thereof. While modern materials such as carbon and other composites are provided to withstand the high braking temperatures encountered, safety considerations preclude use of the aircraft following braking operations for a sufficient period of time to allow the brake discs to cool below a set temperature.
In the aircraft industry, each aircraft has a characteristic turnaround or ground waiting time. This is the time required for the hottest brake disc in the brake disc stack to cool below a specific temperature. For example, several aircraft manufacturers require the temperature of the hottest brake disc to be below the ignition point of the hydraulic brake fluid prior to take-off. The turnaround time characteristic is a function of the aircraft landing speed, mass, brake system characteristics, taxiing procedures, and the like. The turnaround time is, of course, substantially inactive ground time. As such, it is costly in the operation of aircraft, for aircraft generate revenues in the air, not on the ground.
To maximize braking efficiency, it is most desirable to minimize turnaround time and to maximize the brake cooling rate. In other words, it is most desirable to minimize the temperatures that the brake discs reach, while maximizing the rate of heat dissipation within the stack.
In the past, brake disc stacks have been formed with discs of generally uniform thickness. Extensively instrumented tests have shown that discs in the center of the stack cool the slowest, and thereby establish the turnaround time of the associated aircraft. While brake disc material may favorably impact heat dissipation rates, no previous consideration has been given to the thickness and positioning of the discs as a means of achieving such heat dissipation.